There are numerous systems available providing a double or dual-containment assembly including inner or primary pipes contained within outer or secondary containment pipes to transport dangerous or hazardous fluids within the inner pipes. In the event of a leak or emission of fluid or vapors from the inner pipes, the leaking substance is intended to be safely contained within the outer pipes. Historical applications for such double-containment systems are found in the nuclear, gas and petroleum refining, and chemical processing industries. It is also known to provide certain types of detectors and/or drainage devices in the annulus between the inner and outer piping components in the event that there is leakage, for the detection and/or collection of such leakage.
Differential thermal expansion in double-containment systems occurs when the inner and outer piping components expand or contract to different degrees, or at different rates relative to each other. Almost all double-containment systems are subject to changes in temperature during operation, and/or to differences in operating temperature between the inner and outer piping components. This causes differential thermal expansion or contraction of the inner and outer piping components relative to each other, thus causing the inner and outer piping components to move relative to each other.
When the inner piping components expand or contract relative to the outer piping components, and if the inner piping components are installed in an axially unrestrained manner, the deflection of the primary piping due to the expansion or contraction of the primary piping accumulates at the elbow sections of the primary piping. In this case, the inner elbow sections are subjected to bending and/or torsional movements relative to the outer elbow sections. Although elbow fittings by their nature are capable of greater flexibility than comparable straight sections of pipe, when elbow sections are subjected to bending and/or torsional movements, stresses are intensified, and in some instances, this intensification can lead to failure.
In many known double-containment systems, the inner piping components are not permitted to bend or otherwise move either laterally and/or axially relative to the outer piping components, particularly in the area of the elbows. Rather, the straight sections of inner piping are restrained by interstitial supports or other types of fittings within the outer piping components, and/or the inner elbow sections are restrained by fittings relative to the outer elbow sections or are attached to the outer elbow sections, preventing movement of the inner and outer elbow sections relative to each other.
In some known double-containment systems, the inner piping components may be able to move axially relative to the outer piping components, but the elbow fittings do not permit movement of the inner elbow sections relative to the outer elbow sections. The elbow sections become points of restraint, which can lead to failure when there is differential thermal expansion or contraction.
In other known double-containment systems, the inner piping components may be able to move relative to the outer piping components, but only within narrow limits, and once these narrow limits are exceeded, the inner piping components come into contact with the outer piping components. This is frequently the case when standard or off-the-shelf short-radius/short-radius combinations of elbow fittings are employed to make double-containment elbow fittings, which has been a common practise to date in the double-containment industry. Because the centerline radius of the inner elbow section is less than the centerline radius of the outer elbow section, there is less space between the larger-radius surfaces than between the smaller-radius surfaces within the annulus between the inner and outer elbow sections. As a result, when the most common types of differential thermal expansion or contraction occur, i.e., when the inner piping expands toward the outer piping or when the outer piping contracts toward the inner piping, there is contact between the inner and outer piping components. The elbow fittings are therefore not permitted to fully bend or flex in response to differential thermal expansion or contraction, but rather essentially behave as internal anchors, and become points of restraint, which frequently leads to premature failure.
It is typically necessary in double-containment piping systems to provide support for the primary piping by positioning one or more interstitial supports between the primary and outer containment sections of straight pipe, thus employing the structural integrity of the outer containment piping to support the primary piping through such interstitial supports. Typically, multiple interstitial supports are used, and the spacing between the interstitial supports is selected based on the longest span of primary piping that can be allowed before the primary piping sags or deflects beyond a maximum allowable deflection. The degree of deflection of the primary piping depends upon the weight of the primary piping, the weight of the fluid transported through the primary piping, the internal pressure and temperature of the primary piping, the material of construction of the primary piping, and the amount of temperature change experienced during operation of the primary piping. The temperature change is determined based on the temperature condition of the double-containment pipe assembly at the completion of construction in comparison to the high or low temperatures that it will experience when in service.
As described above, almost all chemical and petroleum product piping systems are subject to changes in temperature during operation, and during such temperature changes, there can be relatively substantial expansion and/or contraction of the primary piping relative to the outer containment piping causing the inner piping to move axially, radially and/or laterally relative to the outer containment piping. Typically, the interstitial supports in known double-containment systems do not accommodate for such relative movements, or the selection and/or location of such interstitial supports within the double-containment systems does not adequately compensate for such relative movements. As a result, significant stress is induced within such double-containment systems, which frequently can lead to a rupture or other failure of the primary piping or outer containment piping.
Accordingly, in double-containment systems developed to date, there has been insufficient means (and in many instances no means) for accommodating or alleviating differential thermal expansion and/or contraction of the inner and outer piping components relative to each other, and as a result, such systems have operated as restrained systems, developing large axial stresses, which can lead to failure, and leakage of hazardous fluids or vapors.